Checking post-quantum signatures in Bitcoin Script

What a SHRINCS spend costs under different sets of Script opcodes, measured on an experimental fork's private test network. Not production software. It does not make any coins quantum-safe.

The four scenarios

SHRINCS is a hash-based signature scheme. Checking one signature means recomputing a few thousand SHA256 hashes in a fixed pattern. The question is what Bitcoin Script would need to do that check, and what a spend costs under each answer.

Scenarios A, B, C, C′, E, and E′ were measured with the same checker compiled six ways. A and C were also mined on the test network. Scenario D is computed from the signature size alone. "Stateful" and "stateless" are the two SHRINCS signature types: the small one a wallet normally uses, and the large fallback for when signing state is lost.

Transaction size and fee

One input, two outputs, one signature. Fees are paid per vbyte, and witness bytes count for a quarter, so the whole-transaction byte count and the vbyte count differ. The fee columns assume the shown rates and nothing else.

Stateful signature, 660 bytes

Stateless signature, 5,777 bytes

Reading the two charts: under scenario A the checker program is 95,350 bytes and the spend costs 24,135 vbytes, about 186 ordinary payments. Byte reversal alone cuts the program almost in half. Functions cut it by a further factor of twelve, to 1,416 vbytes, about 11 ordinary payments. The join fix takes 86 vbytes more off. A native opcode would bring it to 279 vbytes, about twice an ordinary payment. For the stateless type the signature itself is 5,777 bytes, so even a native opcode leaves the spend at 1,559 vbytes.

Execution cost

The fork gives every transaction an execution allowance of 10,000 units per weight unit and charges each opcode a fixed price plus data-dependent extras. A spend that exceeds its allowance is invalid. Interpreter time is the median of 7 runs on one machine with counters off; it is indicative, not a benchmark.

One caution for reading across rows. Each program signs its own transaction, because the script is part of the signed message, and a different message changes how much hash-chain work the signature needs. So the charged units in these two tables include signature-to-signature variation of a few percent. The matched-input tables further down remove that variation.

Stateful

Stateless

Three things stand out. The charged work barely changes across A, B, and C: functions and byte reversal compress the program, not the computation. The share of the allowance rises from 2.5% to 36.3% for the stateful spend not because C does more work but because a smaller transaction gets a smaller allowance. And the stateless spend under C uses 59% of its allowance, which leaves little room for anything else in the same transaction.

Nine tenths of the charge is the fixed price of instructions, most of them stack shuffling: PICK, CAT, DROP, small pushes, IF and ENDIF. The SHA256 hashing is under a tenth. The cost model prices this checker as bookkeeping, not cryptography. Programs with a lot of skipped code, as in scenario A, also run slower per charged unit than the model predicts, because parsing skipped code is not charged. Both points bear on any recalibration of the fork's prices.

What OP_MULTI changes

OP_MULTI applies one operation to a run-time number of stack items: push the items, push the count, then OP_MULTI OP_SHA256 hashes them all as one message. The checker builds every hash input by joining parts, so this is where the opcode would help if it helped anywhere.

Building the scenario exposed something else. The audited compiler starts every join from an empty push, which costs two extra opcodes per join. Fixing that needs no new opcode, and it is scenario C′. Scenario E then adds OP_MULTI everywhere it applies. Scenario E′ adds it only for hashing, and only where the cost table says it is cheaper.

These tables use one public test signature per signature type and run every program on it, so the signature bytes are identical across rows and only the program differs. That is the fair comparison; the transaction tables above cannot give it, because each program there signs a different message.

Matched input, stateful

Matched input, stateless

The join fix removes 8.0% of the stateful program bytes and 8.6% of its charged cost; 8.4% and 9.0% for the stateless type. OP_MULTI everywhere then removes a further 1.7% of bytes but adds 1.6% to the charged cost, 1.8% for the stateless type. OP_MULTI where it is cheaper changes the charged cost by -1,119 and -420 units, from 2 and 12 uses.

The reason is in the cost table. The OP_MULTI byte itself is free, but its count is an ordinary push and pays the fixed 1,250, and the opcode then charges the target's fixed price once per logical operation. Against that, a chain of CATs pays 3 units per byte of every intermediate result. So hashing with OP_MULTI wins once the intermediate bytes of a join exceed about 417: three 200-byte parts already cross that line, at 40,630 units against 42,364 on the pinned evaluator. The 16-byte hashes that dominate this checker do not come close, and the few large joins it has, 12 in the stateless program, are too few to matter. OP_MULTI applied to joins that are not hashed, or to cleanups, always pays the count push for nothing under this table.

So the result is specific: a hash-based signature verifier, whose joins are short, does not gain from OP_MULTI. A program that hashes long lists of large items would. All extended programs pass the same 406 acceptance and rejection checks as scenario C.

Throughput

If every transaction on the network used one scenario, how many would fit per second? Same method and assumptions as Jonas Nick's chart: 4,000,000 weight units per block, one block per 600 seconds, an average transaction with 2.27 inputs and 2.64 outputs. The P2TR row reproduces his figure. These are capacity estimates from size, not measured network throughput.

Show as table

Scenario	Witness bytes per input	Tx per second	Tx per block
Today: P2TR key spend	64	6.55	3,929
A. Restoration only, stateful	96,075	0.03	18
B. Restoration + byte reversal, stateful	52,904	0.06	33
C. Restoration + byte reversal + functions, stateful	5,201	0.53	315
C′. C with the join fix, stateful	4,855	0.56	336
E. C′ + OP_MULTI everywhere, stateful	4,789	0.57	340
E′. C′ + OP_MULTI where it is cheaper, stateful	4,849	0.56	336
D. Native SHRINCS opcode (hypothetical), stateful	660	2.81	1,684
A. Restoration only, stateless	139,294	0.02	13
B. Restoration + byte reversal, stateless	126,590	0.02	14
C. Restoration + byte reversal + functions, stateless	19,933	0.14	87
C′. C with the join fix, stateless	18,755	0.15	92
E. C′ + OP_MULTI everywhere, stateless	18,503	0.16	93
E′. C′ + OP_MULTI where it is cheaper, stateless	18,709	0.15	92
D. Native SHRINCS opcode (hypothetical), stateless	5,777	0.48	286

His SHRINCS point sits at 4.13 because it uses a 324-byte signature from a different parameter set. Ours is 660 bytes from the pinned specification. The gap between scenario C and scenario D, about five times, is the cost of checking in Script instead of in the node. The gap between A and C, about seventeen times, is what the fork's two extra opcode groups buy.

Feasibility

Three things apply to every scenario. The output is ordinary Taproot, so its key path remains vulnerable to a quantum computer; a SHRINCS leaf alone does not protect the coins. There is no wallet, no signer state management, and no security proof; the scheme's own specification lists its proof as unfinished. And the numbers come from one experimental fork whose cost table is being recalibrated, so the execution costs will move; the sizes will not.

What was not done

• No comparison with Simplicity. Simplicity is a rival approach, and a recent post put its SHRINCS checker at about 50 kilobytes. The existing Simplicity implementation uses different SHRINCS parameters, so its size cannot be compared with ours yet. Someone has to build both to the same spec first.
• The coins are not quantum-safe. Our outputs use ordinary Taproot, which keeps an old-style key path. Removing that path is a separate consensus change.
• No wallet, no signer, no security proof. This is a checker, running in a lab.

Check it yourself

Everything is in the repository, which is private for now. Its README has a reading guide. The code, the test inputs, the mined transactions, and the measurements are all there, and a clean build on GitHub reproduces the programs and test results byte for byte. This page is generated from the committed reports by docs/build_page.py; the scenario tables come from reports/multi-scenario.json, which the follow-up audit re-derives and re-executes.

Built from reports at commit 33b1f45: multi-scenario.json cce774f31d85, costs.json 29ca417588eb. Sources: SHRINCS specification, the fork, BIP 440, BIP 441, Rusty Russell's 2024 draft, Jonas Nick's throughput script.